The present invention relates to an optical compensatory sheet comprising a transparent support and an optically anisotropic layer provided thereon and formed from discotic liquid crystal molecules. The invention also relates to a liquid crystal display equipped with the optical compensatory sheet.
A liquid crystal display of an STN mode comprises a liquid crystal cell of an STN (Super Twisted Nematic) mode, two polarizing elements placed on both sides of the liquid crystal cell, and one or two optical compensatory sheets (phase retarders) placed between the liquid crystal cell and the polarizing element.
The liquid crystal cell comprises a pair of substrates, rod-like liquid crystal molecules and an electrode layer. The rod-like liquid crystal molecules are provided between the substrates. The electrode layer has a function of applying a voltage to the rod-like liquid crystal molecule. Each of the substrates has an orientation layer, which has a function of aligning the rod-like liquid crystal molecules. The rod-like liquid crystal molecules are twisted with a chiral agent. A twist angle of the molecules is in the range of 180xc2x0 to 360xc2x0. The liquid crystal display of the STN mode can give a clear image with a large capacity according to a time-sharing addressing method even if the display has a simple matrix electrode structure (having no active matrix such as a thin film transistor or a diode).
Without an optical compensatory sheet, a liquid crystal display of an STN mode gives an image colored blue or yellow because of birefringence of rod-like liquid crystal molecules. The blue or yellow color is inconvenient for not only a black and white image but also a color image. An optical compensatory sheet has a function of removing color from an image to display a bright and clear image. The optical compensatory sheet also has a function of enlarging a viewing angle of a liquid crystal cell. As the optical compensatory sheet, a stretched birefringent film has been usually used. Japanese Patent Provisional Publication Nos. 7(1995)-104284 and 7(1995)-13021 disclose an optical compensatory sheet of a stretched birefringent film for a liquid crystal display of an STN mode.
In place of the stretched birefringent film, an optical compensatory sheet comprising an optically anisotropic layer on a transparent support has been proposed. The optically anisotropic layer is formed by aligning discotic liquid crystal molecules and fixing the aligned molecules. The discotic liquid crystal molecules usually have large birefringence, and they also have various alignment forms. Accordingly, an optical compensatory sheet obtained from the discotic liquid crystal molecules has a specific optical characteristic that cannot be obtained by the conventional stretched birefringent film.
B. Mourey et al. disclose a triphenylene discotic liquid crystal molecule having negative birefringence [Mol. Cryst. Liq. Cryst. vol. 84 (1982) pp. 193]. In the case that this discotic liquid crystal molecule is used for an optical compensatory sheet, it is necessary to align the molecules uniformly in the optically anisotropic layer. Accordingly, it is preferred to orient the discotic liquid crystal molecules in monodomain alignment. However, conventional discotic liquid crystal molecules form dual domain alignment, and hence defects are often observed at the interface between the domains. Because of this, conventional discotic liquid crystal molecules hardly give optical characteristics required for an optical compensatory sheet. Since optical characteristics of discotic liquid crystal molecules generally depend on the chemical structure of the molecules, many kinds of discotic liquid crystal molecules have been studied and developed to obtain desired optical characteristics.
Japanese Patent Provisional Publication Nos. 7(1995)-306317 and 9(1997)-104866 disclose 2,3,6,7,10,11-hexa{4-(4-acryloyloxyhexyloxy)benzoyloxy}triphenylene as a discotic liquid crystal compound to form an optically anisotropic layer of optical compensatory sheet.
The retardation (xcex94nd) of optical compensatory sheet is determined according to the optical characteristics of the liquid crystal cell to compensate. The retardation (xcex94nd) is defined as a product of the refractive anisotropy (xcex94n) and the optical thickness (d) of the optically anisotropic layer, and therefore an optically anisotropic layer having a high refractive anisotropy (xcex94n) can satisfyingly compensate the liquid crystal cell even if the thickness (d) is small.
It is very difficult to form an optically anisotropic layer having a sufficiently high refractive anisotropy (xcex94n) from the discotic liquid crystal compound disclosed in Japanese Patent Provisional Publication Nos. 7(1995)-306317 and 9(1997)-104866.
An object of the present invention is to provide an optical compensatory sheet comprising an optically anisotropic layer having a high refractive anisotropy.
The present invention provides an optical compensatory sheet comprising a transparent support, an orientation layer and an optically anisotropic layer formed from discotic liquid crystal molecules represented by the formula (I) in this order, said optically anisotropic layer having a refractive anisotropy in the range of 0.065 to 0.16: 
in which each of A1 and A2 independently is hydrogen, a halogen atom, an alkyl group having 1 to 12 carbon atoms or an alkoxy group having 1 to 12 carbon atoms; Y is hydrogen, a halogen atom, an alkyl group having 1 to 12 carbon atoms, an alkoxy group having 1 to 12 carbon atoms, an acyl group having 2 to 13 carbon atoms, an alkylamino group having 1 to 12 carbon atoms or an acyloxy group having 2 to 13 carbon atoms, or Y is combined with A2 to form a five-membered or six-membered ring; Z is a halogen atom, an alkyl group having 1 to 12 carbon atoms, an alkoxy group having 1 to 12 carbon atoms, an acyl group having 2 to 13 carbon atoms, an alkylamino group having 1 to 12 carbon atoms or an acyloxy group having 2 to 13 carbon atoms; L is a divalent linking group selected from the group consisting of xe2x80x94Oxe2x80x94, xe2x80x94Coxe2x80x94, xe2x80x94Sxe2x80x94, xe2x80x94NHxe2x80x94, an alkylene group, an alkenylene group, an alkynylene group, an arylene group and a combination thereof; Q is a polymerizable group; a is 1, 2, 3 or 4; and b is an integer of 0 to 4-a.
The invention also provides a liquid crystal display comprising a liquid crystal cell of an STN mode, two polarizing plates placed on both sides of the liquid crystal cell, and one or two optical compensatory sheets placed between the liquid crystal cell and the polarizing plate, wherein the optical compensatory sheet comprises a transparent support, an orientation layer and an optically anisotropic layer formed from discotic liquid crystal molecules represented by the formula (I) in this order, said optically anisotropic layer having a refractive anisotropy in the range of 0.065 to 0.16, and said discotic liquid crystal. molecules being aligned at an average inclined angle of 50xc2x0 to 90xc2x0 and being twisted at a twist angle of 90xc2x0 to 360xc2x0.
The invention further provides liquid crystal display comprising a liquid crystal cell of a TN mode, two polarizing plates placed on both sides of the liquid crystal cell, and one or two optical compensatory sheets placed between the liquid crystal cell and the polarizing plate, wherein the optical compensatory sheet comprises a transparent support, an orientation layer and an optically anisotropic layer formed from discotic liquid crystal molecules represented by the formula (I) in this order, said optically anisotropic layer having a refractive anisotropy in the range of 0.065 to 0.16, and said discotic liquid crystal molecules being so aligned that the inclined angle changes according to the distance between the molecule and the plane of the transparent support.
The applicants have studied and found that an optically anisotropic layer formed from discotic liquid crystal molecules represented by the formula (I) has a high refractive anisotropy of 0.065 to 0.16.
The optically anisotropic layer having a high refractive anisotropy compensates a liquid crystal cell sufficiently even if the thickness is relatively small. Therefore, a relatively thin optically anisotropic layer can be used in an optical compensatory sheet. The alignment of discotic liquid crystal molecules can easily be controlled in the thin optically anisotropic layer. Accordingly, the alignment can be adjusted under the optimum conditions.
The alignment of discotic liquid crystal molecules can easily be controlled in the optical compensatory sheet of the invention. Therefore, the sheet can advantageously be used in not only a liquid crystal display of a STN mode (for which the applicants have first studied) but also other displays (e.g., a display of a TN mode) by adjusting the alignment according to the display mode.